Numerical Simulation for Supplemental Type Certification of Aircraft Flying into Known Icing

Heavily externally modified aircraft pose unique challenges in terms of re-certification. This is also true for the icing aspects. Subjects such as handling qualities, regulatory performances, resistance to ice shedding, and air data accuracy degradation are classical subjects to be addressed in the scope of a Supplemental Type Certificate (STC) involving the installation of large fairings or pods and for which there are no limitations for flight into known icing conditions. The demonstration of compliance to the applicable regulations is eased using CFD software with icing accretion computation capabilities. In this paper, we show how such software is currently used at Sabena technics, a French MRO specializing in mission aircraft modifications, as support to external modifications STCs. First, ice accretion is computed on the added external surfaces in line with the applicable regulation recommendations. The maximum ice mass is compared to the one for which the aircraft was certified in terms of bird strike, to show there is no risk of structural damage due to ice shedding from the new fairings or pods. Then, if the airflow around the basic aircraft air data sensors can be affected by the new external modifications, a CFD study is performed to compare the flow characteristics with and without ice accretion; similar characteristics would allow limiting air data accuracy flight tests to the configuration without ice accretion. Finally, a CFD study is performed to compare the impact of the modification on the handling qualities and regulatory performances, with and without ice accretion. Again, a similar impact would allow limiting the associated flight tests to the configuration without ice accretion. If those CFD studies show that ice accretion has a non-negligible impact, then CFD is used to predict the most significant ice shapes, which are then built with specific material and installed on the test aircraft to assess the aircraft in-flight behavior with simulated ice. Only when aircraft behavior is marginal, or confidence in the predicted ice shapes is low, will flight tests with real ice (natural or artificial) be required. Due to the quality of the currently available CFD software, this costly and potentially hazardous step is usually not necessary for the modifications performed at Sabena technics.